Adds Dataset.query() method, analogous to pandas DataFrame.query() (#…

…4984)

* initial work on Dataset.query

* dataset query: test backends, engines, parsers; add docstring

* add error test

* unfortunate typo

* test three dims

* refine tests

* fix error message

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

* add requires decorators

* revert change, should be func name

* improve Dataset.query tests

* add DataArray.query

* add query to API docs

* add query to whats new

* fix black, mypy

* refine test parameterisation and requirements

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

ci/requirements/environment.yml

@@ -22,6 +22,7 @@ dependencies:
   - nc-time-axis
   - netcdf4
   - numba
+  - numexpr
   - numpy
   - pandas
   - pint

doc/api.rst

@@ -138,6 +138,7 @@ Indexing
    Dataset.set_index
    Dataset.reset_index
    Dataset.reorder_levels
+   Dataset.query
 
 Missing value handling
 ----------------------
@@ -321,6 +322,7 @@ Indexing
    DataArray.set_index
    DataArray.reset_index
    DataArray.reorder_levels
+   DataArray.query
 
 Missing value handling
 ----------------------

doc/whats-new.rst

@@ -22,6 +22,10 @@ v0.17.1 (unreleased)
 
 New Features
 ~~~~~~~~~~~~
+
+- Add :py:meth:`Dataset.query` and :py:meth:`DataArray.query` which enable indexing
+  of datasets and data arrays by evaluating query expressions against the values of the
+  data variables (:pull:`4984`). By `Alistair Miles <https://github.com/alimanfoo>`_.
 - Allow passing ``combine_attrs`` to :py:meth:`Dataset.merge` (:pull:`4895`).
   By `Justus Magin <https://github.com/keewis>`_.
 - Support for `dask.graph_manipulation

xarray/core/dataarray.py

@@ -4354,6 +4354,70 @@ def argmax(
         else:
             return self._replace_maybe_drop_dims(result)
 
+    def query(
+        self,
+        queries: Mapping[Hashable, Any] = None,
+        parser: str = "pandas",
+        engine: str = None,
+        missing_dims: str = "raise",
+        **queries_kwargs: Any,
+    ) -> "DataArray":
+        """Return a new data array indexed along the specified
+        dimension(s), where the indexers are given as strings containing
+        Python expressions to be evaluated against the values in the array.
+
+        Parameters
+        ----------
+        queries : dict, optional
+            A dict with keys matching dimensions and values given by strings
+            containing Python expressions to be evaluated against the data variables
+            in the dataset. The expressions will be evaluated using the pandas
+            eval() function, and can contain any valid Python expressions but cannot
+            contain any Python statements.
+        parser : {"pandas", "python"}, default: "pandas"
+            The parser to use to construct the syntax tree from the expression.
+            The default of 'pandas' parses code slightly different than standard
+            Python. Alternatively, you can parse an expression using the 'python'
+            parser to retain strict Python semantics.
+        engine: {"python", "numexpr", None}, default: None
+            The engine used to evaluate the expression. Supported engines are:
+            - None: tries to use numexpr, falls back to python
+            - "numexpr": evaluates expressions using numexpr
+            - "python": performs operations as if you had eval’d in top level python
+        missing_dims : {"raise", "warn", "ignore"}, default: "raise"
+            What to do if dimensions that should be selected from are not present in the
+            Dataset:
+            - "raise": raise an exception
+            - "warning": raise a warning, and ignore the missing dimensions
+            - "ignore": ignore the missing dimensions
+        **queries_kwargs : {dim: query, ...}, optional
+            The keyword arguments form of ``queries``.
+            One of queries or queries_kwargs must be provided.
+
+        Returns
+        -------
+        obj : DataArray
+            A new DataArray with the same contents as this dataset, indexed by
+            the results of the appropriate queries.
+
+        See Also
+        --------
+        DataArray.isel
+        Dataset.query
+        pandas.eval
+
+        """
+
+        ds = self._to_dataset_whole(shallow_copy=True)
+        ds = ds.query(
+            queries=queries,
+            parser=parser,
+            engine=engine,
+            missing_dims=missing_dims,
+            **queries_kwargs,
+        )
+        return ds[self.name]
+
     # this needs to be at the end, or mypy will confuse with `str`
     # https://mypy.readthedocs.io/en/latest/common_issues.html#dealing-with-conflicting-names
     str = utils.UncachedAccessor(StringAccessor)

xarray/core/dataset.py

@@ -7001,5 +7001,78 @@ def argmax(self, dim=None, **kwargs):
                 "Dataset.argmin() with a sequence or ... for dim"
             )
 
+    def query(
+        self,
+        queries: Mapping[Hashable, Any] = None,
+        parser: str = "pandas",
+        engine: str = None,
+        missing_dims: str = "raise",
+        **queries_kwargs: Any,
+    ) -> "Dataset":
+        """Return a new dataset with each array indexed along the specified
+        dimension(s), where the indexers are given as strings containing
+        Python expressions to be evaluated against the data variables in the
+        dataset.
+
+        Parameters
+        ----------
+        queries : dict, optional
+            A dict with keys matching dimensions and values given by strings
+            containing Python expressions to be evaluated against the data variables
+            in the dataset. The expressions will be evaluated using the pandas
+            eval() function, and can contain any valid Python expressions but cannot
+            contain any Python statements.
+        parser : {"pandas", "python"}, default: "pandas"
+            The parser to use to construct the syntax tree from the expression.
+            The default of 'pandas' parses code slightly different than standard
+            Python. Alternatively, you can parse an expression using the 'python'
+            parser to retain strict Python semantics.
+        engine: {"python", "numexpr", None}, default: None
+            The engine used to evaluate the expression. Supported engines are:
+            - None: tries to use numexpr, falls back to python
+            - "numexpr": evaluates expressions using numexpr
+            - "python": performs operations as if you had eval’d in top level python
+        missing_dims : {"raise", "warn", "ignore"}, default: "raise"
+            What to do if dimensions that should be selected from are not present in the
+            Dataset:
+            - "raise": raise an exception
+            - "warning": raise a warning, and ignore the missing dimensions
+            - "ignore": ignore the missing dimensions
+        **queries_kwargs : {dim: query, ...}, optional
+            The keyword arguments form of ``queries``.
+            One of queries or queries_kwargs must be provided.
+
+        Returns
+        -------
+        obj : Dataset
+            A new Dataset with the same contents as this dataset, except each
+            array and dimension is indexed by the results of the appropriate
+            queries.
+
+        See Also
+        --------
+        Dataset.isel
+        pandas.eval
+
+        """
+
+        # allow queries to be given either as a dict or as kwargs
+        queries = either_dict_or_kwargs(queries, queries_kwargs, "query")
+
+        # check queries
+        for dim, expr in queries.items():
+            if not isinstance(expr, str):
+                msg = f"expr for dim {dim} must be a string to be evaluated, {type(expr)} given"
+                raise ValueError(msg)
+
+        # evaluate the queries to create the indexers
+        indexers = {
+            dim: pd.eval(expr, resolvers=[self], parser=parser, engine=engine)
+            for dim, expr in queries.items()
+        }
+
+        # apply the selection
+        return self.isel(indexers, missing_dims=missing_dims)
+
 
 ops.inject_all_ops_and_reduce_methods(Dataset, array_only=False)

xarray/tests/__init__.py

@@ -83,6 +83,7 @@ def LooseVersion(vstring):
 has_cartopy, requires_cartopy = _importorskip("cartopy")
 # Need Pint 0.15 for __dask_tokenize__ tests for Quantity wrapped Dask Arrays
 has_pint_0_15, requires_pint_0_15 = _importorskip("pint", minversion="0.15")
+has_numexpr, requires_numexpr = _importorskip("numexpr")
 
 # some special cases
 has_scipy_or_netCDF4 = has_scipy or has_netCDF4

xarray/tests/test_dataarray.py

@@ -7,6 +7,7 @@
 import numpy as np
 import pandas as pd
 import pytest
+from pandas.core.computation.ops import UndefinedVariableError
 from pandas.tseries.frequencies import to_offset
 
 import xarray as xr
@@ -39,6 +40,7 @@
     requires_dask,
     requires_iris,
     requires_numbagg,
+    requires_numexpr,
     requires_scipy,
     requires_sparse,
     source_ndarray,
@@ -4620,6 +4622,74 @@ def test_pad_reflect(self, mode, reflect_type):
         assert actual.shape == (7, 4, 9)
         assert_identical(actual, expected)
 
+    @pytest.mark.parametrize("parser", ["pandas", "python"])
+    @pytest.mark.parametrize(
+        "engine", ["python", None, pytest.param("numexpr", marks=[requires_numexpr])]
+    )
+    @pytest.mark.parametrize(
+        "backend", ["numpy", pytest.param("dask", marks=[requires_dask])]
+    )
+    def test_query(self, backend, engine, parser):
+        """Test querying a dataset."""
+
+        # setup test data
+        np.random.seed(42)
+        a = np.arange(0, 10, 1)
+        b = np.random.randint(0, 100, size=10)
+        c = np.linspace(0, 1, 20)
+        d = np.random.choice(["foo", "bar", "baz"], size=30, replace=True).astype(
+            object
+        )
+        if backend == "numpy":
+            aa = DataArray(data=a, dims=["x"], name="a")
+            bb = DataArray(data=b, dims=["x"], name="b")
+            cc = DataArray(data=c, dims=["y"], name="c")
+            dd = DataArray(data=d, dims=["z"], name="d")
+
+        elif backend == "dask":
+            import dask.array as da
+
+            aa = DataArray(data=da.from_array(a, chunks=3), dims=["x"], name="a")
+            bb = DataArray(data=da.from_array(b, chunks=3), dims=["x"], name="b")
+            cc = DataArray(data=da.from_array(c, chunks=7), dims=["y"], name="c")
+            dd = DataArray(data=da.from_array(d, chunks=12), dims=["z"], name="d")
+
+        # query single dim, single variable
+        actual = aa.query(x="a > 5", engine=engine, parser=parser)
+        expect = aa.isel(x=(a > 5))
+        assert_identical(expect, actual)
+
+        # query single dim, single variable, via dict
+        actual = aa.query(dict(x="a > 5"), engine=engine, parser=parser)
+        expect = aa.isel(dict(x=(a > 5)))
+        assert_identical(expect, actual)
+
+        # query single dim, single variable
+        actual = bb.query(x="b > 50", engine=engine, parser=parser)
+        expect = bb.isel(x=(b > 50))
+        assert_identical(expect, actual)
+
+        # query single dim, single variable
+        actual = cc.query(y="c < .5", engine=engine, parser=parser)
+        expect = cc.isel(y=(c < 0.5))
+        assert_identical(expect, actual)
+
+        # query single dim, single string variable
+        if parser == "pandas":
+            # N.B., this query currently only works with the pandas parser
+            # xref https://github.com/pandas-dev/pandas/issues/40436
+            actual = dd.query(z='d == "bar"', engine=engine, parser=parser)
+            expect = dd.isel(z=(d == "bar"))
+            assert_identical(expect, actual)
+
+        # test error handling
+        with pytest.raises(ValueError):
+            aa.query("a > 5")  # must be dict or kwargs
+        with pytest.raises(ValueError):
+            aa.query(x=(a > 5))  # must be query string
+        with pytest.raises(UndefinedVariableError):
+            aa.query(x="spam > 50")  # name not present
+
 
 class TestReduce:
     @pytest.fixture(autouse=True)